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This photo, provided by ProtoLife, shows vesicles, artificial membranes for cells, made from scratch. Such synthetic structures could conceivably reproduce themselves, but would that constitute the creation of life?

In November 2011, NASA launched its biggest, most ambitious mission to Mars. The $2.5 billion Mars Science Lab spacecraft will arrive in orbit around the Red Planet this August, releasing a lander that will use rockets to control a slow descent into the atmosphere. Equipped with a "sky crane," the lander will gently lower the one-ton Curosity rover on the surface of Mars. Curiosity, which weighs five times more than any previous Martian rover, will perform an unprecedented battery of tests for three months as it scoops up soil from the floor of the 96-mile-wide Gale Crater. Its mission, NASA says, will be to "assess whether Mars ever was, or is still today, an environment able to support microbial life."

For all the spectacular engineering that’s gone into Curiosity, however, its goal is actually quite modest. When NASA says it wants to find out if Mars was ever suitable for life, they use a very circumscribed version of the word. They are looking for signs of liquid water, which all living things on Earth need. They are looking for organic carbon, which life on Earth produces and, in some cases, can feed on to survive. In other words, they’re looking on Mars for the sorts of conditions that support life on Earth.

But there’s no good reason to assume that all life has to be like the life we’re familiar with. In 2007, a board of scientists appointed by the National Academies of Science decided they couldn’t rule out the possibility that life might be able to exist without water or carbon. If such weird life on Mars exists, Curiosity will probably miss it.

Philosophical challenge
Defining life poses a challenge that’s downright philosophical. There’s no ambiguity in looking for water, because we have a clear definition of it. That definition is the same whether you’re on Earth, on Mars, or in intergalactic space. It is the same whether you’re dealing with water as ice, liquid, or vapor. But there is no definition of life that’s universally agreed upon. When Portland State University biologist Radu Popa was working on a book about defining life, he decided to count up all the definitions that scientists have published in books and scientific journals. Some scientists define life as something capable of metabolism. Others make the capacity to evolve the key distinction. Popa gave up counting after about 300 definitions.

Things haven’t gotten much better in the years since Popa published "Between Necessity and Probability: Searching for the Definition and Origin of Life" in 2004. Scientists have unveiled even more definitions, yet none of them have been widely embraced. But now Edward Trifonov, a biologist at the University of Haifa in Israel, has come forward with a new attempt at defining life, based on a new strategy. Rather than add on yet another definition to the pile, he’s investigating the language that previous scientists have used when they talk about life.

Trifonov acknowledges that each definition of life is different, but there’s an underlying similarity to all of them. “Common sense suggests that, probably, one could arrive to a consensus, if only the authors, some two centuries apart from one another, could be brought together,” he writes in a recent issue of the Journal of Biomolecular Structures and Dynamics (article PDF).

In lieu of resurrecting dead scientists, Trifanov analyzed the linguistic structure of 150 definitions of life, grouping similar words into categories. He found that he could sum up what they all have in common in three words. Life, Trifonov declares, is simply self-reproduction with variations.

Trifonov argues that this minimal definition is useful because it encompasses both life as we know it and life as we may discover it to be. And as scientists tinker with self-replicating molecules, they may be able to put his definition to the test. It may be possible for them to create a system of molecules that meets the requirements. If it fails to come "alive," it will show that the definition was missing something crucial about life.

Trifonov's editors at the journal invited a number of other scientists who study the origin of life to issue their verdict on Trifonov's definition. Judging from their responses, it doesn’t look like anyone’s ready to link arms and sing "Kumbaya" over their beakers of primordial soup. Popa, for example, questions whether the best way out of the definitional bind is to look for consenus. “It does apply very well to fields where basic research has more or less ended, yet it makes it difficult for pioneers and novel theories to gain recognition, irrespective of how right they are,” he writes (article PDF). If science is nothing but a popularity contest, ideas like plate tectonics might have never been discovered and confirmed.

What's missing?
A number of the scientists who responded to Trifonov felt that his definition was missing one key feature or another, such as metabolism, a cell, or information. Eugene Koonin, a biologist at the National Center for Biotechnology Information, thinks that Trifonov’s definition is missing error correction. He argues that “self-reproduction with variation” is redundant, since the laws of thermodynamics ensure that error-free replication is impossible. “The problem is the exact opposite,” Koonin observes: if life replicates with too many errors, it stops replicating. He offers up an alternative: life requires “replications with an error rate below the sustainability threshold.”

Jack Szostak, a Nobel-prize winning Harvard biologist, simply rejects the search for any definition of life. "Attempts to define life are irrelevant to scientific efforts to understand the origin of life," he writes (article PDF).

Szostak himself has spent two decades tinkering with biological molecules to create simple artificial life. Instead of using DNA to store genetic information and proteins to carry out chemical reactions, Szostak hopes to create cells that only contain single-stranded RNA molecules. Like many researchers, Szostak suspects that RNA-based life preceded DNA-based life. It may have even been the first kind of life on Earth, even if it cannot be found on the planet today.

Life, Szostak suspects, arose through a long series of steps, as small molecules began interacting with each other, replicating, getting enveloped into cells, and so on. Once there were full-blown cells that could grow, divide, and evolve, no one would deny that life had come to exist on Earth. But it’s pointless to try to find the precise point along the path where life suddenly sprang into being and met an arbitrary definition. "None of this matters, however, in terms of the fundamental scientific questions concerning the transitions leading from chemistry to biology," says Szostak.

It’s conceivable that Mars has Earthlike life, either because one planet infected the other, or because chemistry became biology along the same path on both of them. In either case, Curiosity may be able to do some good science when it arrives at Mars this summer. But if it’s something fundamentally different, even the most sophisticated machines may not be able to help us until we come to a decision about what we’re looking for in the first place.

This report was originally published by Txchnologist as "Can a Scientist Define 'Life'?" The Txchnologist is a forward-facing magazine presented by GE that takes a look at the wide world of science and technology. The stories and opinions do not necessarily reflect the views of the company.

Copyright General Electric 2012. Reprinted with permission.

Explainer: Six frontiers for alien life

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In "The Day the Earth Stood Still," a remake of the 1951 science-fiction classic, an alien named Klaatu (played by Keanu Reeves, right) visits Earth to save us humans from ourselves. The story is a work of science fiction, with the emphasis on fiction, says Seth Shostak, a senior astronomer at the SETI Institute and a technical adviser on the film. For example, to be able to detect a dangerous buildup of carbon dioxide in our atmosphere and come save us from global warming, an alien that could travel at light speed would have to reside no more than about 50 light-years away. "I doubt that there are any aliens that close," Shostak says. And even if there are, "they might not care about our problems."

Scientific accuracy aside, Shostak says the film could hook a new generation on space science, just as the original film helped direct his career, which is dedicated to the search for E.T. As kids stumble out of the theater, they might ask, do aliens exist?

Click the "Next" arrow above to explore the evidence, from the scientifically plausible to the incredible.

With so many stars, alien life is probable

NASA

Shostak notes that there is no direct proof for any life beyond Earth, but the universe is home to a lot of stars. And as research over the past decade has shown, perhaps at least 50 percent of those stars harbor planets. Shostak estimates there are 1 trillion planets in the Milky Way alone. "Surely some of them have undergone what Earth has undergone and developed life, and eventually what we call sentient life," he says. The argument, he notes, is simply one of probability. "If we are the only intelligent beings in the galaxy, or for that matter in the universe, then we are truly a miracle," he says. This image from the Hubble Space Telescope shows a cluster of young stars in the Milky Way.

Water is a key ingredient for life as we know it. And liquid water, it turns out, is fairly common in our local solar system. For example, evidence is mounting that liquid water may flow underneath the surface of Mars. Europa, a moon of Jupiter, appears to have a liquid ocean. So too might the Jovian moons Callisto and Ganymede. Saturn's moons Titan and Enceladus, shown here, may be watery. Even Venus might have a bit of liquid water in its atmosphere. "There you already have seven other worlds that might have liquid water, just in our backyard. So that's kind of encouraging news," Shostak says.

Life evolved 'quickly' on Earth

Abigail Allwood

Scientists estimate that planet Earth is about 4.5 billion years old. The earliest evidence for life comes from 3.4 billion-year-old mats of bacteria called stromatolites in Australia. Since even bacteria are biologically complex, scientists think they arose from life forms that got a foothold on Earth even earlier. "That suggests it wasn't terribly improbable, the evolution of life, because it happened very quickly," Shostak says. The caveat, of course, is that Earth could have won the evolutionary equivalent of the lottery, and no place else is quite so lucky.

Life thrives in extreme environments

G. Wanger / JCVI / G. Southam /

Almost everywhere scientists go on Earth, they find life: the cold, dark depths of the oceans; snuggled up to piping-hot hydrothermal vents; buried under the Antarctic ice; and in South America's parched Atacama Desert. "Life can adapt to really tough conditions and, of course, most of the universe is going to be filled with habitats that are tough," Shostak says. For example, Mars is a harsh environment, but some of the microbes found on Earth, including the one shown here found deep in a mine, could survive beneath the surface of the Red Planet, he notes. These findings of so-called extremophiles have allowed scientists to scale back their list of requirements for extraterrestrial life. "We just say it has to have some liquid water, and maybe that's it," Shostak says.

Shostak and his colleagues at the SETI Institute frequently harness some of the world's largest radio telescopes to home in on distant stars for a telltale signal of alien communications. Although their searches have raised a few alarms, the signals have been dismissed as human-caused interference, such as noise from a passing satellite. Contact remains elusive. Undaunted, the scientists keep searching. Meanwhile, a signal detected on Aug. 15, 1977, during a search with Ohio State University's Big Ear Observatory, continues to pique interest because it has never been explained. "It was impressive enough to encourage the astronomer who found it to write 'Wow!' on the printout," says Shostak. Follow-up experiments to detect it again, however, have failed. "You can say it was E.T. and then he went off the air. You may never know," Shostak says. "But it is not science to say it was E.T."

Some see evidence that 'aliens' have visited

Justin Norton
/
AP

Somewhere around half the people in the U.S. believe that aliens have already visited us. To back their claims, witnesses have presented snapshots of flying saucers and debris from crash landings. None of the evidence, however, convinces Shostak. Nor does he buy into theories that the world's governments are coordinated and efficient enough to collectively keep what would be the world's biggest secret. "That's hard for me to believe," he says. Such doubt does little to stop the tide of tourists coming to places such as Roswell, N.M., the site of a purported UFO crash more than 60 years ago. This fake alien at a museum is a commonly photographed attraction.